The Environmental Protection Department has already written to companies calling on them to set dials to 25.5 degrees, which overseas studies have found is most people's comfort level. It will follow up with letters to schools, flat-dwellers and public housing estates.

The move follows a study by Polytechnic University showing that temperatures in 90 per cent of Hong Kong's offices average 21 to 23 degrees, with some as cool as 17.

Changing the setting to 25.5 will be one of the themes of World Environment Day on Sunday.

'If air conditioning users who normally set the temperature lower than the suggested level raised it by three degrees, Hong Kong can shave $1 billion off electricity bills,' said the department's assistant director, Eric Chan Sui-wa, after a ceremony to launch No Air Condition Day yesterday.

Mr Chan said air conditioning consumed one-third of the energy used in Hong Kong every year, and was also a major factor in the greenhouse effect blamed for global warming.

More than 100,000 people took an oath yesterday to conserve energy and avoid using air conditioning, and about 174 schools switched from air conditioning to fans, despite 30-degree heat.

Footprint, an environmental protection education concern group, said air conditioning accounted for 70 per cent of electricity use by schools.

The group, which initiated No Air Condition Day, said about 174,000 kW of electricity, worth $139,200, was saved by schools that turned off their air conditioning.

'The air is fresher, as we all opened windows and germs will not be trapped in our classrooms,' said Primary Five student John Tang Siu-kei, 11. 'We tend to fall asleep in class after lunch, as air conditioning makes us feel too comfortable. Being sweaty makes us feel more awake.'